1. Field of the Invention
The present invention relates to an electronic device such as a packaged surface-acoustic-wave (SAW) device, and particularly relates to an electronic device, which has an element chip housed in a ceramic chip carrier and a cap bonded to a sealing ring by using seam welding, thereby achieving hermetic sealing.
2. Description of the Related Art
SAW devices having a packaged SAW element are widely used in many fields, e.g., for filters used for an RF stage or filters used for an IF stage in a mobile communication apparatus. FIG. 1 shows the sectional view of an exemplary packaging configuration of a SAW device (hereinafter referred to as a SAW device 1) as mentioned above, and the configuration thereof will be described below.
The SAW device 1 includes a chip carrier 2 having a base 6, a ceramic layer 7, and a ceramic layer 8, and a SAW element chip 3, a sealing ring 4 having an opening 13, a metal cap 5, a plurality of electrode pads 10, an external electrode 11, a housing space 14 having an opening 9 and an opening 12, and bonding wires 15.
The SAW element chip 3 is hermetically sealed in a package defined by the chip carrier 2, the sealing ring 4, and the metal cap 5. The ceramic layers 7 and 8 are laminated on the base 6 defined by a substantially rectangular ceramic board. The sealing ring 4 is made of Kovar (Fexe2x80x94Coxe2x80x94Ni alloy) and is placed on the ceramic layer 8, which is the topmost layer. The sealing ring 4 is brazed on the ceramic layer 8 with an Ag brazing material. The surface of the sealing ring 4 is usually coated with primary Ni-plating and top-coat Au plating.
The ceramic layer 7 under the ceramic layer 8 has the opening 9, which has a substantially rectangular shape. The plurality of electrode pads 10 is disposed on the ceramic layer 7. The external electrode 11 extends from the ends of the electrode pads 10 over the sides of the ceramic layer 7 and the base 6, and to the bottom surface of the base 6. Generally, the electrode pads 10 and the external electrode 11 are made of a W (tungsten)-metallized electrode coated with primary Ni-plating and top-coat Au plating. The ceramic layer 8 on the ceramic layer 7 also has the opening 12, which has a substantially rectangular shape. Thus, the chip carrier 2 has the housing space 14 formed by the openings 9 and 12 of the ceramic layers 7 and 8. The sealing ring 4 also has the opening 13 formed therein, which has a substantially rectangular shape.
The SAW element chip 3 is housed in the housing space 14, and is fixed to the top surface of the base 6 within the housing space 14 by using an adhesive. Predetermined electrodes of the SAW element chip 3 are electrically connected to the electrode pads 10 by using the bonding wires 15. Subsequently, the electrodes of the SAW element chip 3 are electrically connected to the outside through the external electrode 11.
After the SAW element chip 3 is mounted in the chip carrier 2, the metal cap 5 for sealing is placed on the sealing ring 4 and is seam-welded thereto. Subsequently, the opening 13 is hermetically sealed by the metal cap 5. The metal cap 5 mainly consists of Kovar as does the sealing ring 4, and the surface thereof is coated with Ni-plating. The thickness of the Ni-plating is relatively large and may be as much as 3 xcexcm or more.
Kovar is a metal that has a thermal coefficient of expansion which is similar to that of ceramic. Therefore, the use of Kovar as a main material of the sealing ring 4 and the metal cap 5 can prevent distortion of the chip carrier 2 or a stress crack due to the expansion differences between the sealing ring 4, the metal cap 5, and the chip carrier 2, which may occur in the course of cooling during or after welding.
The base 6, the ceramic layers 7 and 8, and the electrode pads 10 are simultaneously fired and are integrated to define the chip carrier 2. Since the sealing ring 4 is brazed to the top surface of the chip carrier 2 by using the Ag brazing material, the hermeticity between the chip carrier 2 and the sealing ring 4 is maintained. The metal cap 5 is placed on the sealing ring 4 and is seam-welded thereto by using the Ni-plating of the metal cap 5 as the brazing material, whereby the hermeticity between the sealing ring 4 and the metal cap 5 is provided. Since the SAW element chip 3 is hermetically sealed in the package defined by the chip carrier 2, the sealing ring 4, and the metal cap 5 in the above-described manner, the water resistance and the moisture resistance of the SAW device 1 are ensured. Since the metal cap 5 is bonded to the sealing ring 4 by using the Ni-plating of the metal cap 5 without using any other brazing materials, the bonding cost is reduced, and variation of bonding due to application variation of the brazing material is eliminated.
FIG. 2 shows another known SAW device having a package with a configuration that is different from that in the case of the SAW device 1. In this case, the SAW element chip 3 is mounted with its face down, and bumps 16 are electrically connected to electrode pads 17 on the base 6.
In the SAW device 1 shown in FIG. 1, seam-welding is used as to weld the metal cap 5 to the sealing ring 4 so as to achieve hermetic sealing. FIG. 3 illustrates how the metal cap 5 is seam-welded to the sealing ring 4. As shown in this drawing, the metal cap 5 is placed on the sealing ring 4 on the chip carrier 2 in which the SAW element chip 3 is mounted, and a pair of roller electrodes 18a and 18b is placed so as to come in contact with the outer edge of the metal cap 5. Then, welding currents I1+I2 from an alternating-current (AC) power supply pass between the roller electrodes 18a and 18b, which roll and move around the perimeter of the metal cap 5. Since Joule""s heat is generated when the roller electrodes 18a and 18b move around the perimeter of the metal cap 5, the plating of the sealing ring 4 and the Ni-plating of the metal cap 5 are melted and function as brazing materials. Thus, the perimeter of the metal cap 5 is welded to the sealing ring 4, and high hermeticity of the package is achieved.
However, in the above-described cases of the known SAW devices, slight fluctuations of the condition of the welding machines for seam-welding and individual differences between chip carriers can compromise hermeticity of the SAW devices, and can deteriorate yields and reliability for manufacturing the SAW devices.
In order to overcome the problems described above, preferred embodiments of the present invention provide an electronic device, which has an element chip housed in a ceramic chip carrier, a sealing ring provided on the ceramic chip carrier, and a Ni-plated cap welded to the sealing ring so as to close the element chip, and to achieve sufficient hermeticity between the sealing ring and the Ni-plated cap.
According to a preferred embodiment of the present invention, an electronic device includes a ceramic chip carrier having a recess, an element chip provided in the recess, a metal sealing ring which is bonded to the ceramic chip carrier such that the metal sealing ring surrounds an opening of the recess of said ceramic chip carrier, and a metal cap closing the metal sealing ring to hermetically seal the recess, the metal cap being coated with Ni-plating having a thickness of about 1 xcexcm to about 2 xcexcm for bonding the metal cap to the metal sealing ring.
Preferably, the metal cap of the electronic device is seam-welded to the metal sealing ring by the Ni-plating, which is used as a brazing material.
When the thickness of the Ni-plating is as thin as about 1 xcexcm to 2 xcexcm, the metal cap and the metal sealing ring are firmly bonded, and the hermeticity, the water resistance, and the moisture resistance therebetween are greatly improved. Accordingly, a SAW device with high reliability can be manufactured. Further, still higher hermeticity can be achieved by electroless plating.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.